1. Field of the Invention
The present invention generally relates to electric lamps, and more particularly, to electric lamps having an improved filter for decreasing the attractiveness of the lamp to night insects.
2. Description of Related Art
Lamps used in applications where it is undesirable for light to attract insects, commonly referred to as "bug lights" or "bug lamps", typically have a colored filter or pigment that blocks a portion of the radiation emitted by the lamp that is most visible to night insects. The filter generally absorbs or attenuates radiation having a wavelength of about 450 nanometers or less, which includes ultraviolet and blue portions of the spectrum of light emitted by the lamp. An acceptable level of light visible to humans, which are more sensitive to wavelengths in the yellow portion of the spectrum than the ultraviolet and blue portions of the spectrum, is maintained because radiation having a wavelength greater than about 450 nanometers is substantially unaffected or transmitted through the filter.
Prior art bug lamps have typically utilized various cadmium sulfide (CdS) pigments to provide decorative and functional "yellow" bug lamps. For example, U.S. Pat. No. 3,320,460, the disclosure of which is expressly incorporated herein in its entirety, discloses a electrostatically applied coating of light-scattering pigmented powder including a cadmium compound. While the CdS pigments are very effective, cadmium is considered a hazardous material. Therefore, non-Cd pigments have more recently been used even though they are generally less effective at blocking the undesired portions of the spectrum of radiation emitted by the lamp. Examples of non-Cd pigments include Sb doped rutile pigment as a diffuse coating on the inside of a lamp envelope and an organic dye in a silicone coating. Another example of a non-Cd pigment is a diffuse coating of a fine-ground blend of praseodymium doped zircon (Zr:Pr) and silica electrostatically applied to the inside of a lamp envelope such as disclosed in U.S. Pat. No. 5,107,167, the disclosure of which is expressly incorporated herein in its entirety. A further example of a non-Cd pigment is a luminescent phosphor coating such as yttrium aluminum garnet doped with cerium (YAG:Ce) applied to the inside of a lamp envelope such as disclosed in U.S. Pat. No. 5,118,985, the disclosure of which is expressly incorporated herein in its entirety.
Lamps having a relatively high operating temperature, such as for example linear quartz halogen lamps and metal halide lamps, are widely used for exterior lighting. The prior art pigments or coatings, however, cannot be used on such high temperature lamps. The prior art coatings do not have coefficients of thermal expansion closely matching quartz or other materials used on high temperature lamp jackets and therefore are not compatible. Additionally, the prior art coatings are applied to the inside of the lamp envelope and thus would effect the chemistry of lamps such as quartz halogen and metal halide. Furthermore, the prior art coatings do not have the abrasion resistance or durability to be applied to the exterior of the lamp. Accordingly, there is a need for a coating or filter that has desired optical absorption characteristics, has high temperature stability, has the ability to withstand the temperature conditions and thermal cycling experienced over a typical operating life of the lamps, has a binder system compatible with quartz, that can be applied to the outside of the lamp, and that is relatively inexpensive to apply.